The invention relates to a process for producing a heat-conducting connection between two workpieces.
For structures with highly heat-producing workpieces, for example workpieces in the form of electrical or electronic components, good heat-conducting connecting technology is necessary for heat dissipation.
Conventionally the components are soldered. The temperature stability and thermal fatigue resistance of solder is however very limited.
EP-0 242 626 A2 (GR 86 P 1242) discloses a process for producing a heat-conducting connection between a workpiece in the form of an electronic component and a workpiece in the form of a substrate, in which a paste is applied to the component and/or the substrate; the paste consists of a mixture of metal powder which sinters at the sintering temperature and sinters both to the component and also to the substrate, and of a liquid.
The paste is dried and after drying of the paste the component and the substrate are heated together or separately to a temperature which is at least 100xc2x0 C., but is still below the sintering temperature. This heating is expressly unpressurized heating according to the document.
At latest after this heating the component is placed on the completely dried paste of the substrate. The entire arrangement is then heated to the sintering temperature with simultaneous application of mechanical pressure of at least 900 N/cm2.
This known process is especially suited for large-area, power semiconductors which have been produced in MOS technology, major advantages however also being achieved in the production of other electronic components as well.
In the older German patent application 100 09 678.6 (GR 98 E 1895) which had not been published previously a process for producing a heat-conducting adhesive joint between two workpieces is proposed in which first of all a porous sintered layer of heat-conducting material, i.e. one which is penetrated with cavities like a sponge, especially of silver, is produced and is located between the two workpieces and is surface-sintered to each workpiece.
This sintered layer is filled with liquid hardenable cement which afterwards wets each workpiece. Filling takes place for example by sucking the liquid cement into the cavities of the sintered layer which act as capillaries.
Afterwards the cement is hardened and the adhesive joint is completed.
The sintered layer is produced such that a paste is applied to one workpiece and/or the other workpiece; the paste consists of a mixture of a powder which sinters at the sintering temperature and which sinters to each of the two workpieces at this temperature, and of a liquid; the two workpieces are brought together such that the paste is located between the two workpieces and makes contact between the surfaces of the two workpieces, afterwards the paste is dried and the dried powder is sintered by heating to the sintering temperature.
High density and thus good thermal conductivity of the sintered layer of heat-conducting material can also be achieved with the step of applying a certain mechanical pressure to the powder during the sintering process or after completion of this process.
Preferably a powder is used which is chosen from a group of metals, especially precious metals and semiprecious metals.
It is especially advantageous if a silver powder is used and sintering of this powder is carried out in an oxidizing atmosphere, since to sinter this powder a sintering temperature between 100xc2x0 C. and 250xc2x0 C. is advantageously sufficient. Sintering in an oxidizing atmosphere can however also be advantageous for sinterable powders which contain substances different from silver.
It is also mentioned that a high density and thus good thermal conductivity of the sintered layer of heat-conducting material can be achieved by applying a certain mechanical pressure to the powder during the sintering process or after completion of this process.
With the proposed process, larger workpieces can be advantageously joined to surfaces to be joined which are larger than 1 cm2, for example 2xc3x972 cm2 or more, or even more, to one another securely over the entire surface.
The object of the invention is make available a process for producing an adhesive-free, heat-conducting connection between two workpieces, with which larger workpieces which have surfaces to be joined which are larger than 1 cm2, for example 2xc3x972 cm2 or even more, can be connected to one another with high strength and very good heat conduction on these surfaces in their entirety.
This object is achieved by the features of claim 1.
According to this approach the process as claimed in the invention has the following steps:
Producing a porous sintered layer of heat-conducting material which is located between the two workpieces and is sintered superficially to each workpiece, and
Subsequent compaction of the porous sintered layer which has been sintered to the two workpieces by pressing the two workpieces relatively against one another.
The porous sintered layer of heat-conducting material, i.e. one which is penetrated with cavities like a sponge, is produced preferably and advantageously by the following steps:
Application of a paste to one workpiece and/or the other workpiece which consists of a mixture of a powder of heat-conducting material which sinters at the sintering temperature and sinters to each of the two workpieces at this temperature, and a liquid,
Bringing together the two workpieces such that the paste is located between the two workpieces and makes surface contact between the two workpieces,
Drying of the paste, and
Sintering of the dried powder by heating to the sintering temperature.
Preferably and advantageously a powder of heat-conducting material is used which sinters at a sintering temperature of at most 250xc2x0 C. and moreover sinters to each of the two workpieces. Preferably a powder is used which is chosen from a group of metals, especially precious and semiprecious metals, and which sinters at the sintering temperature of at most 250xc2x0 C. and moreover sinters to each of the two workpieces. Use of a sinterable metal powder which has silver is especially advantageous.
It is especially advantageous if the sintering itself and sintering of the powder to each workpiece are carried out at the sintering temperature in an oxidizing atmosphere. This applies especially when using a sinterable metal powder which has silver.
For subsequent compaction of the porous sintered layer which has been sintered to the two workpieces it is advantageous to use mechanical pressure which is chosen to be so high that as many cavities and pores of the porous sintered layer as possible, best all of them, are closed, but the two workpieces are not damaged.
It is especially advantageous if the subsequent compaction of the porous sintered layer which has been sintered to the two workpieces is carried out by pressure sintering. Pressure sintering here means applying the mechanical pressure used for subsequent compaction to the porous sintered layer with simultaneous heating of the porous sintered layer to the sintering temperature which can be the same or different from the sintering temperature used in the production of the porous sintered layer.
Pressure sintering has the advantage that the pores of the sintered layer close more easily and the strength of the connection produced by sintering together between the sintered layer and the workpieces can be increased even more.
For later compaction of the porous sintered layer which has been sintered to the two workpieces, feasibly and especially in pressure sintering a pressure of at least 900 N/cm2 is used. An increase of this pressure to 1000 N/cm2 or to 1500 cm2 and more can be advantageous.
In the process as claimed in the invention, by producing the porous sintered layer which has been sintered to the workpieces a connection between the workpieces is produced which is strengthened by the subsequent compaction of the sintered layer especially by pressure sintering such that a high-strength, very good heat-conducting connection between the workpieces is formed, which can have a large area, especially larger than 2xc3x972 cm2, and is especially well suited for attachment of electronic components, especially power semiconductor components such as for example IGBTs, MOS-FETs, diodes, thyristors, etc., which during operation produce high power losses which must be efficiently dissipated in order not to exceed the maximum operating temperature.
The process as claimed in the invention moreover has other major advantages especially compared to the process known from EP-0 242 626 A2, including:
a) Simple positioning and fixing of a host of parts on a substrate, for example for multichip modules, power converters or the like;
b) Surface irregularities are filled and cause fewer problems, for example segregations on chips or roughness of the substrate. Thickness fluctuations and irregularities due to screen pressure, template pressure or spraying of metalxe2x80x94especially a silver layerxe2x80x94are eliminated and the density of the pressed sintered layer is more homogeneous.
c) At a lower sintering temperature the stiffness of the sinter bridges formed first in the sintered layer is still low according to the process as claimed in the invention, in this way at the same sintering pressure a higher compaction is achieved than in pressing the silver layer which has been sintered scratch-proof according to EP-0 242 626 A2 (see also the dissertation of Sven Klaka: xe2x80x9cA low temperature connecting technology for building power semiconductor modulesxe2x80x9d, Cuvillier Verlag, Goettingen 1997). The xe2x80x9csofterxe2x80x9d sintered layer according to the process as claimed in the invention is protected against damage by the workpieces between which it is located.
d) The sintering temperature used in the production of the sintered layer of the process as claimed in the invention essentially determines the connection temperature at which the workpieces are connected to one another without the mechanical tension due to different coefficients of thermal expansion. It can remain clearly below the sintering temperature which is used in pressure sintering.
The invention is detailed by way of example in the following description using the drawings.